Simulcast paging systems are well known in the art of paging communication systems. For example, U.S. Pat. No. 5,369,682, assigned to the same assignee as the present invention and incorporated herein by reference, discloses a digital simulcast paging system. In general, such a system includes a paging switch connected to the public switched telephone network, and a plurality of base stations (also referred to as paging stations or transmitters). A caller wishing to page a subscriber of the paging system calls the paging switch using the public switched telephone network. The paging switch then formulates a page to the subscriber and distributes the page to each of the paging base stations. The paging base stations then simultaneously broadcast (simulcast) the page. The subscriber receives the page through a personal paging unit (or "pager") that the subscriber carries.
Paging base stations may include the capability of transmitting pages according to multiple paging protocols. Further, pages encoded under different protocols may be time division multiplexed to increase throughput and decrease the system's costs. Thus, for example, modern paging transmitters are capable of transmitting pages according to a variety of protocols, including, for example, POCSAG.TM. and FLEX.TM. protocols. In this regard, each protocol requires that data is transmitted with a predetermined frequency deviation from a given center frequency. Typically, pagers designed for POCSAG.TM. require a frequency deviation of .+-.4500 Hz. Pagers designed for FLEX.TM. require a frequency deviation of .+-.4800 Hz.
In a simulcast paging system, difficulty can arise in those geographic areas that can receive signals from more than one paging station. Such geographic areas are commonly referred to as "overlapping areas". FIG. 1 depicts two base stations, base station #1 and base station #2. The circles represent the physical area covered by transmissions from each of the two base stations, respectively. The common area defined by the circles constitutes an overlapping area. To ensure continuous coverage over a large geographic area, base stations are positioned in such a way as to guarantee an overlapping area between adjacent base stations.
The problem associated with overlapping areas in a paging system involves two aspects of paging: frequency synthesis and simulcasting. With respect to frequency synthesis, pagers tune to a specific frequency to "listen" for their pages. The present generation of exciters used in a paging transmitter can synthesize radio frequencies with an accuracy of better than 1 Hz. Simulcasting describes the simultaneous manner in which a transmitted paging signal leaves the antenna of all base stations in a paging system. Simulcasting is required for continuous paging coverage in overlapping areas. Presently, the capabilities of simulcast paging systems can ensure the simultaneous transmission of paging data from base stations to within a microsecond.
The precision and accuracy of frequency synthesis and simulcasting, however, leads to a problem present in the overlapping areas in a paging system. More specifically, they cause patterns of time-invariant constructive and destructive RF interference. This time-invariant RF interference is sometimes referred to as standing waves. In a zone of destructive interference, it will be appreciated that a pager could fail to receive a page. The problems present in overlapping areas is further discussed in "System integration of the FLEX.TM. paging protocol," (Parts I & II) by Lee 1. Williams, available from Glenayre Electronics, Inc., Charlotte, N.C.
To eliminate the likelihood of lost pages, a protocol-dependent frequency offset is introduced between adjacent base stations. The magnitude of the offset is approximately a few hundred hertz in typical applications. The offset reduces the duration of the destructive interference to a predetermined length of time, ensuring that a pager will not miss a page due to the standing wave phenomenon. In this way, the problems associated with time-invariant RF interference are effectively eliminated.
Frequency offset and frequency deviation together constitute "mode information" corresponding to a particular protocol. Each protocol has an associated "mode setting" that is unique to it. The accuracy of the mode setting is necessary for the optimal performance of the paging system. In particular, implementing the correct mode setting better guarantees the receipt of pages by pagers.
Base stations are sometimes designed so that they can broadcast pages according to various protocols having an associated mode setting. In this way, a base station can transmit information not only to pagers of a certain protocol, but also to additional pagers of other protocols. One conventional approach for mixing broadcasts of different protocols at a predetermined carrier frequency involves compromise. Both protocols are mixed together on one channel. To broadcast under different protocols, the optimal mode setting values for each protocol are not used. Rather, compromise values for the mode setting are selected in an attempt to suit all relevant protocols. For example, if the POCSAG.TM. and FLEX.TM. protocols are used, one deviation frequency and one offset frequency would be chosen to accommodate both protocols.
Significant drawbacks result from this conventional approach. Although the use of compromise mode settings in this manner provide adequate paging some of the time, the reception of pages in overlapping areas is far from guaranteed. In fact, even the reception of pages in non-overlapping areas is unreliable. Because they often require the exact mode settings they are designed to receive, pagers, depending on this brand and protocol, cannot routinely receive the pages directed to them under this conventional approach.
Another conventional approach for mixing protocols has been used extensively. In this approach, a multi-channel exciter is typically used in a base station. Normally, only a single pair of offset frequency and deviation frequency, i.e., one mode setting, could be defined for one channel in the exciter. Accordingly, only one protocol with its associated mode setting could correspond to a channel. In the event that, for example, two protocols were to be mixed at a given carrier frequency, two channels in the exciter had to be used. Although the carrier frequency for each protocol was identical, the mode settings still had to be customized for each of the two protocols.
Broadcasting one protocol per channel in this conventional way provided multi-mode operation. For example, suppose the POCSAG.TM. protocol is to be transmitted on a first channel in the exciter of the base station and the FLEX.TM. protocol is to be transmitted on a second channel. In operation, assume that the transmitter is keyed up on the first channel to transmit the POCSAG.TM. protocol. To transmit the FLEX.TM. protocol, the transmitter must key down the first channel, implement a channel change, and key up the second channel. Exciters used in this conventional approach can sometimes expend up to approximately 300 ms to implement a channel change.
Substantial disadvantages also plague this conventional approach. Managing a base station under this approach is both time consuming and labor intensive for paging service providers. Furthermore, each protocol's need for a separate channel effectively wastes channels and thus substantially limits the capability of base stations to transmit pages at various frequencies.
In addition to these limitations, an even more fundamental problem is presented. It will be appreciated that the upper limit on the number of subscribers for a given carrier frequency is dictated by the time needed to transmit paging information. Reducing the transmission time of paging data allows paging service providers to carry more subscribers on the given carrier frequency and attendantly increase their revenue. In this regard, the approximately 300 ms wasted to implement a protocol change during multi-mode operation constitutes a significant time period wherein no paging data can be transmitted. During this time no revenue can be produced. This sacrifice of potential revenue-producing time can amount to thousands of dollars of lost revenue per month for a paging service provider. Over time, the loss of revenue-producing time can be significant.
It is apparent that the above-mentioned and other conventional approaches to provide mixing protocols in a paging system are unsatisfactory. The prior art methods are both unreliable and unduly expensive. The present invention is a reliable and cost effective way for implementing multi-mode operation in a paging system.